AS 17681991NZS/AS 17681991

Australian StandardNew Zealand Standard

Lightning protection

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AS 17681991/NZS/AS 17681991

This Standard was prepared under a joint arrangement by Standards Australia and theStandards Association of New Zealand. It was approved for publication on behalf of theCouncil of Standards Australia on 18 September 1991 and on behalf of the Standards Councilof New Zealand on 6 September 1991. It was published on 9 December 1991.

The following organizations are represented on the Committees responsible for thisStandard:

Standards Australia Committee EL/24, Protection Against LightningAssociation of Consulting Engineers AustraliaAustralian Corrosion AssociationAustralian Electrical and Electronic Manufacturers AssociationAustralian Institute of PetroleumBuilding Owners and Managers Association of AustraliaConfederation of Australian IndustryDepartment of DefenceDepartment of Minerals and Energy, N.S.W.Department of Administrative ServicesAustralian Construction ServicesElectricity Supply Association of AustraliaInstitution of Engineers AustraliaPublic Works Department, N.S.W.Railways of Australia CommitteeTelecom AustraliaUniversity of QueenslandStandards Association of New Zealand

Standards Association of New Zealand Electrotechnical Board, 60/, was responsiblefor coordinating the New Zealand participation.

Review of Standards. To keep abreast of progress in industry, Joint Australian/New ZealandStandards are subject to periodic review and are kept up to date by the issue of amendmentsor new editions as necessary. It is important therefore that Standards users ensure that they arein possession of the latest edition, and any amendments thereto.

Full details of all Joint Standards and related publications will be found in the StandardsAustralia and Standards New Zealand Catalogue of Publications; this information issupplemented each month by the magazines The Australian Standard and Standards NewZealand, which subscribing members receive, and which give details of new publications,new editions and amendments, and of withdrawn Standards.

Suggestions for improvements to Joint Standards, addressed to the head office of eitherStandards Australia or Standards New Zealand, are welcomed. Notification of anyinaccuracy or ambiguity found in a Joint Australian/New Zealand Standard should be madewithout delay in order that the matter may be investigated and appropriate action taken.

This Standard was issued in draft form for comment in Australia as DR 90070 and inNew Zealand as DZ 6110.

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AS 17681991NZS/AS 17681991

Australian StandardNew Zealand Standard

Lightning protection

In AustraliaFirst published as AS MC11969.Revised and redesignated AS 17681975.Second edition 1983.Third edition 1991.

In New ZealandFirst published as NZS/AS 17681991.

PUBLISHED JOINTLY BY:STANDARDS AUSTRALIA(Standards Association of Australia), 1 The Crescent, Homebush,NSW, Australia

STANDARDS NEW ZEALANDLevel 10, Standards House,155 The Terrace,Wellington 6001 New Zealand

ISBN 0 7262 7132 2

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PREFACE

This Standard is issued as a joint Standard under the terms of the Memorandum ofUnderstanding between Standards Australia and the Standards Association of New Zealandwith the objective of reducing technical barriers to trade between the two nations. It wasprepared by the Standards Australia Committee on Protection against Lightning and, inAustralia, it supersedes AS 17681983.This Standard is intended to provide authoritative guidance on the principles and practice oflightning protection for a wide range of structures and systems, but excludes those owned oroperated by public utilities and statutory authorities. It is not intended for mandatoryapplication but, if called up in a contractual situation, compliance with this Standard requirescompliance with all relevant clauses of the Standard. Alternative methods of protection tothose described in this Standard will be the subject of future consideration.In general, it is not economically possible to provide total protection against all the possibledamaging effects of lightning, but the recommendations in this Standard will reduce theprobability of damage to a low level, and will minimize any lightning damage that doesoccur. Guidance is given to methods of enhancing the level of protection against lightningdamage, if this is required in a particular situation.Following a review of submissions relating to AS 17681983, several changes and additionshave been made to this Standard. Information is given on the protection of persons andequipment within buildings from the harmful effects of lightning strikes to the building, or toelectrical power or communication services entering the building from remote sites. Revisedrecommendations are given relating to the compatibility of materials used in lightningprotection systems, especially from the point of view of minimizing galvanic corrosion ofcomponents. In addition, changes have been made to recommendations for protection of thesides of tall buildings.Unless it has been specified that lightning protection must be provided, the first decision tomake is whether the lightning protection is needed. Section 2 provides guidance to assist inthis decision. Section 3 provides advice on the protection of persons from lightning, mainlyrelating to the behaviour of persons when not inside substantial buildings. Once a decision ismade that lightning protection is necessary, Section 4 will provide details on interceptionlightning protection for the building or structure. This includes information on the size,material, and form of conductors, the positioning of air terminations and downconductors,and the requirements for the earth terminations. Persons and equipment within buildings canbe at risk from the indirect effects of lightning and Section 5 gives recommendations on theprotective measures that may need to be applied.Section 6 describes methods of lightning protection of various items not covered in earliersections, such as communications aerials, chimneys, boats, fences, and trees. A new clausehas been included on methods for protecting domestic dwellings, where a completeprotection system may not be justified, but some protection is considered desirable.Section 7 sets out recommendations for the protection of structures with explosive or highlyflammable contents. Section 8 gives advice on inspecting, testing, and maintaining lightningprotection systems.A number of appendices are included which provide additional information and advice. Theappendices form an integral part of this Standard unless specifically stated otherwise, i.e.appendices identified as informative only provide supportive or background informationand are therefore not an integral part of this Standard.

Copyright STANDARDS AUSTRALIA/STANDARDS NEW ZEALANDUsers of Standards are reminded that copyright subsists in all Standards Australia and Standards New Zealand publications and software. Except where theCopyright Act allows and except where provided for below no publications or software produced by Standards Australia or Standards New Zealand may bereproduced, stored in a retrieval system in any form or transmitted by any means without prior permission in writing from Standards Australia or StandardsNew Zealand. Permission may be conditional on an appropriate royalty payment. Australian requests for permission and information on commercialsoftware royalties should be directed to the head office of Standards Australia. New Zealand requests should be directed to Standards New Zealand.

Up to 10 percent of the technical content pages of a Standard may be copied for use exclusively inhouse by purchasers of the Standard withoutpayment of a royalty or advice to Standards Australia or Standards New Zealand.Inclusion of copyright material in computer software programs is also permitted without royalty payment provided such programs are usedexclusively inhouse by the creators of the programs.

Care should be taken to ensure that material used is from the current edition of the Standard and that it is updated whenever the Standard is amended orrevised. The number and date of the Standard should therefore be clearly identified.The use of material in print form or in computer software programs to be used commercially, with or without payment, or in commercial contracts is subjectto the payment of a royalty. This policy may be varied by Standards Australia or Standards New Zealand at any time.

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CONTENTSPage

SECTION 1 SCOPE AND GENERAL 1.1 SCOPE 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 APPLICATION 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 REFERENCED DOCUMENTS 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 DEFINITIONS 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 2 ANALYSIS OF NEED FOR PROTECTION 2.1 NEED FOR PERSONAL PROTECTION 7. . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 NEED FOR PROTECTION OF BUILDINGS AND CONTENTS 7. . . . . . . 2.3 NEED FOR PROTECTION OF PERSONS AND EQUIPMENT WITHIN

BUILDINGS 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 3 BEHAVIOURAL PRECAUTIONS FOR PERSONAL SAFETY 3.1 SCOPE OF SECTION 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 PERSONAL CONDUCT 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 EFFECT ON PERSONS AND TREATMENT FOR INJURY BY

LIGHTNING 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 4 PROTECTION OF BUILDINGS 4.1 SCOPE OF SECTION 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 ZONES OF PROTECTION 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 METHODS OF PROTECTION 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 MATTERS TO BE CONSIDERED WHEN PLANNING

PROTECTION 18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 MATERIALS 21. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 FORM AND SIZE OF CONDUCTORS 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7 JOINTS 24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8 FASTENERS 24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9 AIR TERMINATIONS 24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10 DOWNCONDUCTORS 26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.11 TEST LINKS 27. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.12 EARTH TERMINATIONS 27. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13 EARTHING ELECTRODES 29. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.14 METAL IN AND ON A STRUCTURE 30. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 5 PROTECTION OF PERSONS AND EQUIPMENT WITHINBUILDINGS

5.1 SCOPE OF SECTION 33. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 NEED FOR PROTECTION 33. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 MODES OF ENTRY OF LIGHTNING IMPULSES 33. . . . . . . . . . . . . . . . . . 5.4 GENERAL CONSIDERATIONS FOR PROTECTION 34. . . . . . . . . . . . . . . . 5.5 PROTECTION OF PERSONS WITHIN BUILDINGS 36. . . . . . . . . . . . . . . . 5.6 PROTECTION OF EQUIPMENT 37. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 6 PROTECTION OF MISCELLANEOUS STRUCTURES ANDPROPERTY

6.1 SCOPE OF SECTION 42. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 STRUCTURES WITH RADIO AND TELEVISION AERIALS 42. . . . . . . . . 6.3 STRUCTURES NEAR TREES 42. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 PROTECTION OF TREES 42. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 CHIMNEYS, METAL GUYWIRES OR CABLES 43. . . . . . . . . . . . . . . . . . . 6.6 PROTECTION OF MINES 43. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7 PROTECTION OF BOATS 44. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.8 FENCES 45. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.9 MISCELLANEOUS STRUCTURES 46. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.10 PROTECTION OF HOUSES AND SMALL BUILDINGS 47. . . . . . . . . . . . .

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SECTION 7 PROTECTION OF STRUCTURES WITH EXPLOSIVE ORHIGHLYFLAMMABLE CONTENTS

7.1 SCOPE OF SECTION 48. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 GENERAL CONSIDERATIONS 48. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7.3 AREAS OF APPLICATION 48. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 EQUIPMENT APPLICATION 48. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 SPECIFIC OCCUPANCIES 49. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION 8 INSTALLATION AND MAINTENANCE PRACTICE 8.1 WORK ON SITE 53. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 INSPECTION 53. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 TESTING 53. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 RECORDS 53. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 MAINTENANCE 53. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

APPENDICES A THE NATURE OF LIGHTNING AND THE PRINCIPLES OF

LIGHTNING PROTECTION 54. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B NOTES ON EARTHING ELECTRODES AND MEASUREMENT OF

EARTH IMPEDANCE 66. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C THE CALCULATION OF LIGHTNING DISCHARGE VOLTAGES AND

REQUISITE SEPARATION DISTANCES FOR ISOLATION OF ALIGHTNING PROTECTION SYSTEM 77. . . . . . . . . . . . . . . . . . . . . . . . . . .

D WAVESHAPES FOR ASSESSING THE SUSCEPTIBILITY OFEQUIPMENT TO TRANSIENT OVERVOLTAGES DUE TOLIGHTNING 84. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E ALTERNATIVE DETERMINATION OF INDEX E BASED ONLIGHTNING FLASH DENSITY/ENERGY DATA 88. . . . . . . . . . . . . . . . . .

F REFERENCED DOCUMENTS 94. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5 AS 17681991/NZS/AS 17681991

STANDARDS AUSTRALIA/STANDARDS ASSOCIATION OF NEW ZEALAND

Australian/New Zealand StandardLightning protection

SECTION 1 SCOPE AND GENERAL

1.1 SCOPE This Standard sets out guidelines for the protection of persons and property from hazardsarising from exposure to lightning. The recommendations specifically cover the following applications:(a) The protection of persons, both outdoors, where they may be at risk from the direct effects of a

lightning strike, and indoors, where they may be at risk indirectly as a consequence of lightningcurrents being conducted into the building.

(b) The protection of a variety of buildings or structures, including those with explosive orhighly-flammable contents, and mines.

(c) The protection of sensitive electronic equipment (e.g. facsimile machines, modems, computers) fromovervoltages resulting from a lightning strike to the building or its associated services.

The nature of lightning and the principles of lightning protection are discussed and guidance is given toassist in a determination of whether protective measures should be taken.The recommendations in this Standard do not apply to the protection of large scale power orcommunications systems, nor do they apply to the protection of special structures such as oil and gasplatforms.

1.2 APPLICATION This Standard does not override any statutory requirements but may be used inconjunction with such requirements.Compliance with the recommendations of this Standard will not necessarily prevent damage or personalinjury due to lightning but will reduce the probability of such damage or injury occurring.1.3 REFERENCED DOCUMENTS The documents referred to in this Standard are listed in Appendix F.

1.4 DEFINITIONS For the purpose of this Standard, the definitions below apply.1.4.1 Air terminationa conductor or rod of a lightning protection system, positioned so as to intercepta lightning discharge, which establishes a zone of protection.1.4.2 Air termination networka network of air terminations and interconnecting conductors whichforms the part of a lightning protection system which is intended to intercept lightning discharges.1.4.3 Base conductors (base tapes)conductors placed around the perimeter of a structure near groundlevel interconnected to a number of earth terminations to distribute the lightning currents amongst them.1.4.4 Bond (bonding conductor)a conductor intended to provide electrical connection between thelightning protection system and other metalwork and between various metal parts of a structure or betweenearthing systems.1.4.5 Downconductora conductor which connects an air termination with an earth termination.1.4.6 Earth impedance (Z)the electrical impedance of an electrode or structure to earth, derived fromthe earth potential rise divided by the impulse current to earth causing that rise. It is a relatively complexfunction and depends on(a) the resistance component (R) as measured by an earth tester;(b) the reactance component (X), depending on the circuit path to the general body of earth; and(c) a modifying (reducing) time-related component depending on soil ionization caused by high current

and fast rise times.1.4.7 Earth potential rise (EPR)the increase in electrical potential of an earth electrode or earthedstructure, with respect to distant earth, caused by the discharge of current to the general body of earththrough the impedance of that electrode or structure.1.4.8 Earthing boss (terminal lug)a metal boss specially designed and welded to process plant, storagetanks, or steelwork to which earthing conductors are attached by means of removable studs and nuts orbolts.1.4.9 Earthing conductorthe conductor by which the final connection to an earth electrode is made.

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AS 17681991/NZS/AS 17681991 6

1.4.10 Earthing electrodes (earth rods or ground rods)those portions of the earth termination whichmake direct low resistance electrical contact with the earth.1.4.11 Earthing resistancethe resistance of the lightning protection system to the general mass of earth,as measured from a test point.1.4.12 Earth termination (earth termination network)that part of a lightning protection system whichis intended to discharge lightning currents into the general mass of the earth. All parts below the lowesttest link in a downconductor are included.1.4.13 Explosive gas atmospherea mixture of flammable gas, vapour or mist with air in atmosphericconditions in which, after ignition, combustion spreads throughout the unconsumed mixture that is betweenthe upper and lower explosive limits.

NOTE: The term refers exclusively to the danger ari sing from ignition. Where danger from other causes such as toxicity,asphyxiation, and radioactivity may arise this is specifically mentioned.

1.4.14 Finiala term not used in this Standard owing to its confusion with architectural application butoccasionally used elsewhere in other Standards as referring to short vertical air terminations.1.4.15 Hazardous areaan area where an explosive atmosphere is, or may be expected to be presentcontinuously, intermittently or due to an abnormal or transient condition (see the AS 2430 or NZS 6101series).1.4.16 Jointa mechanical and electrical junction between two or more portions of a lightning protectionsystem.1.4.17 Lightning flash (lightning discharge)an electrical discharge in the atmosphere involving oneor more electrically charged regions, most commonly in a cumulonimbus cloud, taking either of thefollowing forms:(a) Ground flash (earth discharge) a lightning flash in which at least one discharge channel reaches the

ground.(b) Cloud flash a lightning flash in which the discharge channels do not reach the earth.1.4.18 Lightning flash densitythe number of lightning flashes of the specified type occurring on or overunit area in unit time. This is commonly expressed as per square kilometre per year (km 2 year1). Theground flash density is the number of ground flashes per unit area and per unit time, preferably expressedas a long-term average value.1.4.19 Lightning protection systema system of conductors and other components used to reduce theinjurious and damaging effects of lightning.1.4.20 Lightning strikea term used to describe the lightning flash when the attention is centred on theeffects of the flash at the attachment point, rather than on the complete lightning discharge.1.4.21 Lightning strike attachment pointthe point on the ground or on a structure where the lower endof the lightning discharge channel connects with the ground or structure.1.4.22 Lightning strokea term used to describe an individual current impulse in a complete groundflash.1.4.23 Side flasha discharge occurring between nearby metallic objects or from such objects to thelightning protection system or to earth.1.4.24 Striking distance (d s)the distance between the tip of the downward leader and the eventual strikeattachment point at the moment of initiation of an upward intercepting leader.1.4.25 Structure or objectany building or construction, process plant, storage tank, tree, or similar, onor in the ground.1.4.26 Surge arrestora protective device, usually connected between any conductor of a system andearth, which limits surge voltages by diverting surge current to earth when a given voltage is exceeded.1.4.27 Test linka joint designed and situated so as to enable resistance or continuity measurements tobe made.1.4.28 Thunderdaya calendar day during which thunder is heard at a given location. The internationaldefinition of lightning activity is given as the number of thunderdays per year (also called isocerauniclevel or ceraunic level).1.4.29 Zone of protectionthe portion of space within which an object or structure is considered to beprotected by a lightning protection system.

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7 AS 17681991/NZS/AS 17681991

SECTION 2 ANALYSIS OF NEED FOR PROTECTION

2.1 NEED FOR PERSONAL PROTECTION A hazard to persons exists during a thunderstorm. Eachyear, a number of persons are struck by lightning particularly when outdoors in an open space such as anexposed location on a golf course, or when out on the water. Others receive electric shocks attributableto lightning when indoors.In built-up areas protection is frequently provided by nearby buildings, trees, power lines or street lightingpoles. Persons within a substantial structure are normally protected from direct strikes, but may be exposedto a hazard from conductive materials entering the structure (e.g. power, telephone, or TV antenna wires)or from conductive objects within the structure which may attain different potentials. Measures for theprotection of persons within buildings or structures are set out in Section 5.Lightning strikes direct to a person or close by may cause death or serious injury. A person touching orclose to an object struck by lightning may be affected by a side flash, or receive a shock due to step, touchor transferred potentials, as described in Appendix A.When moderate to loud thunder is heard, persons out of doors should avoid exposed locations and shouldseek shelter or protection in accordance with the guidance for personal safety provided in Section 3,particularly if thunder follows within 15 s of a lightning flash (corresponding to a distance of less than5 km).

2.2 NEED FOR PROTECTION OF BUILDINGS AND CONTENTS2.2.1 Factors governing decision whether or not to protect A decision to provide lightning protectionmay be taken without any risk assessment, for example, where there is a desire that there be no avoidablerisk. In such cases a clear statement should be made that a lightning protection system should be installedin accordance with this Standard.The object of this Clause is to give guidance on those factors which are capable of assessment in termsof the likelihood of the structure being struck and the consequences of any such strike. The use of thestructure, the nature of its construction, the value of the contents, and the prevalence of thunderstorms inthe area can all be considered in making the assessment.Where it is thought that the consequential effects will be small and that the effect of a lightning flash willmost probably be merely slight damage to the fabric of the structure, it may be economic not to incur thecost of protection but to accept the risk. Even though this decision is made, it is suggested that acalculation is still worth making so as to give some idea of the magnitude of the risk that is being taken.The variety of structures is so great that any method of assessment may lead to anomalies and those whohave to decide on protection should use their judgement. For example, a steel-framed building may befound to have a low risk but as the addition of an air termination and earthing system will give greatlyimproved protection, the small extra cost of doing so may often be worthwhile.A low risk value may arise for chimneys made of brick or concrete. However, where such chimneys arefree-standing or where they project for more than 5 m above the adjoining structure, they will requireprotection regardless of the value of the risk index.In determining how far to go in providing lightning protection for specific cases, or whether it is neededat all, it is necessary to take into account a number of factors. With some structures there will be littledoubt as to the need for protection; examples of such structures are(a) those in or near which large numbers of persons congregate;(b) those concerned with the maintenance of essential public services;(c) those in areas where lightning is prevalent;(d) very tall or isolated structures; and(e) structures of historic or cultural importance.Although structures of large area are more likely to be struck than smaller ones, the cost-effectiveness ofstructure protection is not strongly dependent on this characteristic for non-flammable structures. However,the need to protect electronic equipment and to protect persons against potential differences associated withmetallic services increases with the building area (see Section 5).Any structure which is entirely within a zone protected by an adjacent object or objects (whether protectedor not) should be deemed to be protected (see Clause 4.2).2.2.2 Risk index In Tables 2.1 to 2.5, index figures are given opposite headings denoting the relativedegree of importance or risk in each case.

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AS 17681991/NZS/AS 17681991 8

The risk index, R, is obtained from the equation:R = A + B + C + D + E .... 2.2.2

the index figures A, B, C, D and E being obtained from the tables. The higher the risk index the greater the needfor protection, and vice versa.Table 2.6 shows an assessment of the risk associated with various values of the risk index, R. The table alsoprovides guidance on the need for protection.Examples of risk index calculations for different structures are given in Table 2.7.The risk index equation has been determined empirically. The equation has been applied to a variety of cases and,despite the incompleteness of present knowledge of lightning phenomena, it has been found to lead torecommendations which, in general, accord with commonly accepted practice in Australia.Attention is called to the above factors and their importance without pre-empting the purchasers right todetermine whether or not lightning protection should be provided. However, to avoid ambiguity where a purchaserspecifies lightning protection in accordance with this Standard and provides no further guidance, protection shouldbe provided wherever the risk index, evaluated as described in this Clause, is equal to or greater than 13. Thefactors are set out in Clauses 2.2.3 to 2.2.5.2.2.3 Value and nature of building and contents The value and nature of the building and contents areobviously vital factors in deciding whether the expense of protection is warranted. In addition to direct lossescaused by fires, damage to equipment and buildings and killing of livestock, indirect losses such as interruptionto business services and farming should also be taken into account when assessing the need for protection (seealso Clause 2.3).2.2.4 Relative exposure The relative exposure of a particular building will be an element in determining whetherthe expense of protection is warranted. In closely built-up towns and cities, the hazard is not so great as in theopen country. In the latter, farm buildings are in many cases the most prominent targets for lightning in a largearea. In hilly or mountainous districts, a building upon high ground is usually subject to a greater hazard thanone in a valley or otherwise sheltered area.2.2.5 Frequency and severity of thunderstorms The frequency of occurrence of thunderstorms variessignificantly depending on location. Moreover, the severity of lightning storms, as distinct from their frequencyof occurrence, is known to be much greater in some areas than in others. Hence, the need for protection variesacross the country, although not necessarily in direct proportion to thunderstorm frequency. A few severethunderstorms in a season may make the need for protection greater than a relatively large number of storms oflesser activity.Data on the average yearly distribution of days with thunderstorm activity are given(a) in Figure 2.1 for Australia; and(b) in Figure 2.2 for New Zealand.Thunderday information is of limited usefulness in assessing the need for protection but may be the onlyinformation available on which such an assessment can be made.Lightning detection systems are in use at a limited number of sites in Australia. Such systems detect the numberof ground flashes within a specified area and, in some cases, the peak current of each discharge, thus providinga more meaningful indication of the lightning activity at a given location.Ground flash data covering a region of South East Queensland and North East New South Wales are providedin Appendix E. A procedure for determining a value of the index E in Equation 2.2.2, based directly on groundflash data rather than thunderday data (see Table 2.5), is under consideration. See details given in Appendix E.2.3 NEED FOR PROTECTION OF PERSONS AND EQUIPMENT WITHIN BUILDINGS As explainedin Clause 2.1, persons and equipment within buildings can be at risk from lightning currents and associatedvoltages which may be conducted into the building as a consequence of a lightning strike to the building orassociated services. Some equipment (e.g. electronic equipment, including computers) is especially susceptibleto damage from overvoltages in the electricity supply caused by lightning and such damage may occur even whenthe lightning strike is remote from the building, e.g. from a surge conducted into the building via the electricitysupply.Measures may therefore need to be taken to protect persons and equipment within buildings and Section 5provides further advice on this subject. The measures recommended in Section 5 can be implemented even whena lightning protection system for the building structure has not been provided.The decision as to whether to provide protection specifically directed to equipment will depend on the valueplaced on that equipment and on the cost and inconvenience which might result from the equipment being outof service for an extended period.The risk index determined from Clause 2.2.2 will provide guidance on the likelihood of a building being subjectto a lightning strike with consequent risk of damage occurring to equipment within the building. However, sincedamage to equipment can result from lightning strikes to adjacent properties or to power or signal lines somedistance away, the index value may not be a sufficient indicator of the risk. The incidence of damage occurringto similar equipment within buildings in the vicinity may provide a better guide to the need to protect.

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TABLE 2.1INDEX FIGURE A (TYPE OF STRUCTURE)Usage and contents Value of index A

Protection not justi fied having regard to nature of building, occupancy andcontents 10

Structure and contents inert , occupation infrequent, e.g. domestic outbuilding,farm shed, roadside hoarding, metal chimney or mast 0

Structure containing ordinary equipment or a small number of people,e.g. domestic dwell ing, store, shop, small factory, railway station, tent ormarquee

1

Structure or contents of fair importance, e.g. water tower, store with valuablecontents, office, factory or residential building, non-metallic chimney or mast 2

Cinema, church, school, boat, historical monument of medium importance,densely populated marquee 3

Museum, art gallery, stadium, entertainment complex, telephone exchange,computer centre, aircraft hangar, airport terminal, airport control tower,lighthouse, industrial plant, power station, historical monument or tree ofmajor importance

4

Petrol and gas installation, hospital 5Explosives building 15

TABLE 2.2INDEX FIGURE B (CONSTRUCTION)Construction Value of index B

Fully metallic structure, electr ically continuous 0Reinforced concrete or steel frame with metall ic roof 1Reinforced concrete or steel frame with concrete or other non-metall ic roofCottage or small building of timber or masonry with metall ic roof 2Large area building of timber or masonry with metall ic roofSmall building of timber or masonry with non-metall ic roof 3Large area building of timber or masonry with non-metall ic roofLarge tent or marquee of flammable materialMembrane structures with metall ic frames

4

TABLE 2.3INDEX FIGURE C (HEIGHT)

Height of structure, m Value of index CExceeding Not exceeding06

12

61217

023

172535

253550

456

5070

100

70100140

789

140200

200 1011

TABLE 2.4INDEX FIGURE D (SITUATION)

Situation Value of index DOn the flat, at any elevation 0Hillside up to three-quarters of the way up, or mountainous country up to1000 m 1Mountain top above 1000 m 2

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TABLE 2.5INDEX FIGURE E (LIGHTNING PREVALENCE)

Average thunderdays per year* Value of index EExceeding Not exceeding

024

248

012

81632

163264

345

64 6* See thunderday data in Figures 2.1 and 2.2.NOTE: See Appendix E for an alternative procedure, which is stil l under development, for the determination of theindex E based on ground flash data in lieu of thunderday data.

TABLE 2.6ASSESSMENT OF RISK AND NEED FOR PROTECTION

Risk index, R(R = A + B + C + D + E) Assessment of risk Need for protection

14 Very great Essential

TABLE 2.7EXAMPLES OF THE CALCULATIONS FOR EVALUATING THE

NEED FOR PROTECTION

Example

Index values

Assessmentof risk Protection

A B C D ER = A + B

+ C + D+ E

Type ofstructure Construction Height Situation Prevalence

(Table 2.1) (Table 2.2) (Table 2.3) (Table 2.4) (Table 2.5)10 m high domesticdwelling, brick walls,non-metall ic roof locatedon hillside15 thunderdays

1 3 2 1 3 10 Negligible Not needed

15 m high domesticdwelling, brick walls,non-metall ic roof locatedon hillside30 thunderdays

1 3 3 1 4 12 Fair Might beadvisable

20 m high historictree on flat60 thunderdays

3 3 4 0 5 15 Very great Essential

15 m high aircrafthangar, steel frame withmetall ic roof, located inhilly country at 1000 m15 thunderdays

4 1 3 2 3 13 Medium Advisable

10 m high church, brickwalls, metal roof, locatedon hillside30 thunderdays

3 2 2 1 4 12 Fair Might beadvisable

24 m high office building,reinforced concrete, locatedon flat15 thunderdays

2 2 4 0 3 11 Small Not needed

40 m high office building,reinforced concrete, locatedof flat30 thunderdays

2 2 6 0 4 14 Great Stronglyadvisable

16 m high wooden mastedyacht on opensea10 thunderdays

3 3 3 0 3 12 Fair Might beadvisable

20 m high brick chimneylocated on flat30 thunderdays

2 3 4 0 4 13 Medium Advisable

10 m high marqueelocated on flat40 thunderdays

3 4 2 0 5 14 Great Stronglyadvisable

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NOTES:1 Contours on the map join locations having the same number of thunderdays per year, a thunderday being a day on which thunder

is heard.2 A colour copy of this map is available from the Bureau of Meteorology.

FIGURE 2.1 AVERAGE ANNUAL THUNDERDAY MAP OF AUSTRALIA

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NOTES:1 Contours on the map join locations having the same number of thunderdays per year, a thunderday being a day on which thunder

is heard.2 The above data are based on information contained in the Meteorological Off ice Note No. 82, Frequency of Thunderstorms in

New Zealand, published by the New Zealand Meteorological Service.

FIGURE 2.2 AVERAGE ANNUAL THUNDERDAY MAP OF NEW ZEALAND

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SECTION 3 BEHAVIOURAL PRECAUTIONS FOR PERSONAL SAFETY

3.1 SCOPE OF SECTION This Section provides guidance for personal safety during thunderstorms andmainly applies to behaviour when outdoors.Measures for the protection of persons which should be incorporated in lightning protection systems for buildingsand structures are outlined in other sections.

3.2 PERSONAL CONDUCT Persons seeking protection from lightning should observe the followingprecautions:(a) Seek shelter in a substantial building with at least normal headroom or within a totally enclosed,

metal-bodied vehicle. Conventional fabric tents offer no protection; small sheds offer uncertain protection.(b) If on open ground, remote from shelter, crouch down, singly, with feet together. Footwear or a layer of any

non-absorbing material, such as a plastics sheet, offers some protection against ground currents, should therebe a nearby lightning flash.

(c) If in an open boat keep a low profile. Additional protection is gained by anchoring under relatively highobjects such as jetties and bridges, provided that no direct contact is made with them. Avoid isolated buoysand pylons.

(d) Avoid riding horses or bicycles, or riding in any open vehicle such as a tractor or beach buggy, or in anyenclosed vehicle with a non-metallic roof.

(e) Avoid swimming or wading.(f) Persons in an exposed position during the approach of a thunderstorm are advised to seek shelter. If the time

interval between a lightning flash and hearing the thunder becomes less than 15 s, move quickly to aprotected location as there is immediate danger of a lightning strike nearby.

(g) Avoid high ground and isolated trees. If the vicinity of a tree cannot be avoided, seek a position just beyondthe spread of the foliage.

(h) Avoid touching or standing close to tall metal structures, wire fences and metal clothes lines.(i) Avoid handling substantial metallic objects, and remove metal objects from the hair or head covering.(j) Limit the use of telephones when a thunderstorm is overhead.(k) Avoid contact with electrical appliances and metal objects, e.g. stoves, refrigerators, metal window frames,

sinks, radios and television sets.(l) If the use of household appliances or the telephone is unavoidable, keep clear of other appliances and metal

objects, and keep any such use brief.3.3 EFFECT ON PERSONS AND TREATMENT FOR INJURY BY LIGHTNING* The severity of theinjuries inflicted on a person by lightning depends on the fraction of the total lightning current that flows throughthe persons body and the path of the current through or over the body. The worst situation is where the personis struck on the upper part of the body, so all the current must flow through the trunk, where the heart and lungsare the vitally significant organs, or over the skin. A less dangerous situation is where the person is subjectedto step or touch potentials, and only a small fraction of the total current passes through the body, although thepathway taken by this fraction is still important.The effects of lightning include burns to the skin, which are usually superficial, damage to various bodily organsand systems, unconsciousness, but, most dangerously, cessation of breathing and cessation of heart beat.Independently of these electrically related effects, temporary or permanent hearing impairment may beexperienced as a consequence of the extremely high sound pressure levels associated with a nearby lightningstrike.In the first-aid treatment of a patient injured by lightning, it is essential that breathing be restored by artificialrespiration and blood circulation be restored by external cardiac massage, if appropriate. These procedures shouldbe continued until breathing and heart beat are restored, or it can be medically confirmed that the patient is dead.It should also be noted that the usual neurological criteria for death may be unreliable in this situation. There isno danger in touching a person who has been struck by lightning.Lightning strike victims are sometimes thrown violently against an object, or are hit by flying fragments of ashattered tree, so first-aid treatment may have to include treatment for traumatic injury.Subsequent treatment of a lightning strike patient is a specialized area with important differences from thetreatment of injuries inflicted by electric power current. For example, the nature of the burns, and the extent ofdamage to underlying muscle tissue tends to be severe with electric power current, but mild with lightningcurrent. Neurological and cardiac injuries also are different, and follow different courses.

* For a more comprehensive treatment of the subject covered by this Clause, see the following publication:ANDREWS C.J., COOPER M.A., DARVENIZA M. and MACKERRAS D. (Eds) Lightning injury: Electrical, medical and legalaspects . CRC Press. Baton Range, Florida. (In publication.)

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SECTION 4 PROTECTION OF BUILDINGS

4.1 SCOPE OF SECTION This Section sets out recommendations for installation practices and for the selectionof equipment to prevent or to minimize damage or injury which may be caused by a lightning discharge. Therecommendations apply generally to the protection of buildings and structures. Recommendations for theprotection of particular structures and property are given later in this Standard.

4.2 ZONES OF PROTECTION4.2.1 Basis of recommendations Some parts of a structure are exposed to direct lightning strikes while otherparts lie within zones of protection established by higher parts of the structure.Protection against direct lightning strikes is achieved by installing a lightning protection system in such a waythat its air terminations establish zones of protection enclosing the whole structure.The recommendations that follow are based on the rolling sphere technique of determining zones of protection.Using this technique a sphere of specified radius is theoretically brought up to and rolled over the total building.All sections of the building which the sphere touches are considered to be exposed to direct strokes. Sections ofthe building which cannot be touched by the sphere are considered protected by other sections of the building.A sphere of 45 m radius has been selected to provide a high degree of protection to conventional buildings, thisbeing designated as standard protection. A sphere of smaller radius may be used to establish zones of protectionwhere a higher degree of protection is desired.

NOTE: A sphere of 20 m radius is recommended for the protection of structures with explosive or highly flammable contents (seeClause 7.2.2).

The influence of variation in sphere radius on the protection provided is discussed in Paragraph A7, Appendix A.For unusual or complex building forms, the rolling sphere technique may be used directly in determining bothzones of protection and air termination configurations.4.2.2 Required protection Air terminations should be installed on parts of the structure most likely to be strucksuch as the outermost edges of the roof (especially the corners of an elevated roof), at tops of towers, and onparapets, ridges and chimneys which protrude above the general roof level, in accordance with Clause 4.9.2.The zones of protection established by air terminations on higher parts of a structure should be determined havingregard to the following:(a) Air terminations which do not exceed 45 m above ground are considered to protect lower sections of

structure where these lie in the space beneath an arc of 45 m radius and where the arc passes through thehighest point of the building and is tangential to the ground (see Figure 4.1(a)).

(b) Air terminations or structures in excess of 45 m are considered to protect only those lower sections of thestructure which lie in the space beneath an arc of 45 m radius which is tangential both to the air terminationor side of the building and to the ground, as shown in Figure 4.1(b).For buildings in excess of 45 m, direct strikes to the side of the structure above the 45 m level may beanticipated. However, these are less probable than strikes to the top of the building and are also likely tobe of a lesser magnitude.

Roofs of structures and protruding parts of structures which do not lie within the zones of protection establishedby air terminations on higher parts of the structure should be protected by additional air terminations.Air terminations of height h above a flat roof or horizontal plane are considered to protect points on that planeup to a horizontal distance r from a horizontal air termination conductor or to a horizontal radius r from a verticalair termination rod, where r is given by:

r = (90h h2) . . . . 4.2where r and h are in metres.A simple array of such vertical rods at spacing distances d metres from the nearest adjacent rods on a flat roofor horizontal plane is considered to protect the whole surface within the boundary of the array provided thatd r 2.Table 4.1 and Figure 4.2 illustrate the protective zones established by air terminations on a flat non-conductingroof with a parapet on one side.

4.3 METHODS OF PROTECTION4.3.1 Structural steel-framed buildings Buildings with structural steel framing may be protected by theinstallation of metal air terminations at the high parts of the building, the air terminations being connected to thesteel framing and the framing earthed in the vicinity of the foundation. A typical system is shown in Figure 4.3(see also Clause 4.14.1).

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TABLE 4.1HEIGHT AND SPACING OF AIR TERMINATIONS TO PROTECT A FLAT ROOF

metres

Height of air termination Horizontal distance for which roofis protected Maximum spacing distance for array

h r d0.51.02.0

6.79.4

13.3

9.513.318.8

4.08.0

18.525.6

26.236.2

FIGURE 4.1 ZONE OF PROTECTION ESTABLISHED BY AIR TERMINATIONSON THE HIGHER PARTS OF A STRUCTURE

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NOTE: The hatched areas show the zones of protection established by each air termination. In the top view the zonesare in the plane of the roof.

FIGURE 4.2 ZONES OF PROTECTION ON A FLAT ROOF

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4.3.2 Buildings without structural steel frames4.3.2.1 General The required conditions of protection for non-metallic buildings are generally met by placingmetal air terminations on the uppermost parts of the building or its projections, with conductors connecting theair terminations to each other and to ground. By this means a relatively small amount of metal properlypositioned and distributed can be made to afford a satisfactory degree of protection and, if desired, the materialmay be placed so as to give minimum interference to the appearance of the building. A typical system is shownin Figure 4.4. Additional methods utilizing the individual characteristics of particular types of buildingconstruction are given in Clauses 4.3.2.2 to 4.3.2.4, and in Figure 4.3.4.3.2.2 Structures with continuous metal Structures containing continuous metal, e.g. metal within a roof, wall,floor or covering may, if the amount and arrangement of the metal is adequate in terms of the recommendationsof Clauses 4.10 to 4.14, utilize such metal as part of the lightning protection system.4.3.2.3 Metal-roofed buildings For buildings which are roofed, or roofed and clad, with metal, it may bepossible to dispense with some air terminations and to cater for any upper portions of the building which aresusceptible to damage by earthing such metal.4.3.2.4 Reinforced concrete buildings The following recommendations apply to the use of steel reinforcementin reinforced concrete buildings as part of the lightning protection system (see also Paragraph A5.5.2,Appendix A):(a) General As far as possible, the steel reinforcement should be made electrically continuous in all concrete

elements having a structural purpose, e.g. columns, beams and also in non-structural concrete elements, e.g.concrete wall panels, where the element, or a part of it, if dislodged, could endanger persons below.Where steel reinforcing elements are not in physical contact with each other, lightning discharges may causecracks in the vicinity of the gaps in reinforcement. Where insulating gaps cannot be avoided, the buildingshould be treated in the same way as one of non-conducting materials.Where the steel reinforcement is used as the downconductor system, an effective electrical connection shouldbe made from the air termination system to the steel reinforcement at the top of the building. Suchconnections should be made, by means such as welding or clamping, to the vertical and horizontal bars inas many places as necessary to ensure a multiplicity of conductive paths for the discharge of lightningcurrent.NOTE: Steel reinforcement which is overlapped and tied by means of wire is not considered to provide an effective electricalconnection for this purpose but such joints are acceptable elsewhere as part of the downconductor system where current sharing isassured.

Modern reinforced concrete structures frequently involve several structural techniques including in situreinforced concrete, prestressed reinforced concrete and precast concrete; recommendations for these arelisted in Items (b), (c) and (d).

(b) In situ reinforced concrete The metal rods in the columns of a reinforced concrete structure cast in situ areoccasionally welded at splice points, thus providing definite electrical continuity. Where very tall columnsare involved, a spliced connection between rods is frequently achieved by a mechanical clamping device orthreaded ferrule which also provides a high degree of electrical continuity. Most frequently, however, therods are tied together by steel tie wire at splice points, but despite the fortuitous nature of the metallicconnection, the very large number of rods and crossing points assures a subdivision of the total lightningcurrent into a multiplicity of parallel discharge paths. Experience shows that with this splicing technique therods can also be readily utilized as part of the lightning protection system without thermal or mechanicaldamage to the structure. Particular recommendations on the size, material or number of tie wires are notgiven in this Standard, normal building practice being relied upon to provide adequate continuity.Normal building practice also ensures the multiple conducting paths continue into the building foundations(see Note). The foundations are deep in the mass of earth and the resistivity of concrete is generallycomparable with that of clay or other moderately conductive ground. Hence, except in soils of lowresistivity, the resistance to ground from the foundation reinforcement is often lower than can be obtainedeconomically with driven rods, because of the much greater surface area. Concrete foundations themselvesconstitute a satisfactory earth termination network but their use, as such, precludes the inclusion of baseconductors. It is desirable, however, that a metallic connection to the reinforcing be installed, in a positionsuitable for the bonding of metallic services associated with the building.NOTE: Conductive paths may not be ensured if special building techniques are used, e.g. grouting reinforcing bars into drilled holesin concrete after it has set, using an insulating epoxy-based material.

(c) Prestressed reinforced concrete Prestressed reinforced concrete is used most commonly in the horizontalstructural elements in a building, such as the beams and floor slabs, and only rarely in vertical elements suchas columns. Consequently, the principal reason for avoiding insulating gaps in prestressed concrete relatesto side flashing rather than to the ability of the reinforcement to carry a lightning discharge to ground.Details of the treatment of prestressed concrete in order to avoid side flashing are given in Clause 4.14, andthe principles described in that clause should be used in the rare instance where vertical prestressed elements,such as prestressed columns, occur in a building.

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Although prestressed concrete affords a large reduction in the cross-sectional area of steel reinforcementcompared with conventionally reinforced concrete, calculations indicate that prestressed cables of 10 mmdiameter or larger, will not be damaged thermally by lightning and that thermal effects become negligiblewhen several cables are connected in parallel.

(d) Precast concrete Where electrical continuity is required through precast concrete elements, the structuralconnection details, e.g. attachment plates, threaded ferrules, bolt or dowel connections, should be carefullyexamined from an electrical continuity standpoint. In most cases, the attachment device will be a metallicone and continuity can be achieved by simply welding the attachment device to electrically continuousreinforcement within the precast concrete element.

4.3.3 Structures with flammable or explosive atmosphere Structures in which very small induced sparkspresent an appreciable element of danger, such as structures which contain explosive atmospheres of flammablevapour or gas and structures in which easily ignitable fibres or materials producing combustible flyings are stored,e.g. cotton, grain or explosives, usually require much more than the standard protection. Such structures can beprotected by tall conducting masts earthed at the bottom end, by bonding as detailed in Clause 4.14.2.2, or byoverhead earthed wires (for further details see Section 7).

4.4 MATTERS TO BE CONSIDERED WHEN PLANNING PROTECTION4.4.1 Structures to be erected For structures that are to be erected, the matter of lightning protection should beconsidered in the planning stage, as the necessary measures can often be effected in the architectural featureswithout detracting from the appearance of the building. In addition to the aesthetic considerations, is usually lessexpensive to install lightning protection during construction than afterwards.4.4.2 Design considerations4.4.2.1 General considerations The structure or, if the structure has not been built, the drawings, should beexamined with due regard to all the relevant details of this Standard and in particular to the following:(a) Metal used in the roof, walls, framework or reinforcement above or below ground, e.g. sheet piling, to

determine the suitability of such metal in place of, or for use as, components of the lightning protectionsystem.NOTE: For a non-metallic roof, the position of any conduit, piping, water mains or other earthed metal immediately beneath the roofshould be noted, as this may inadvertently attract a discharge if not shielded by an adjacent roof or structure, or downconductor onor above the roof.

(b) Available positions for downconductors providing the required number of low impedance paths from the airtermination network to the earth termination; this is particularly important for internal downconductors.

(c) The nature and resistivity of the soil as revealed by trial bore holes for foundation purposes or soil resistivitytests with, where economically practicable, the driving and testing of a trial earth rod electrode with theobject of designing a suitable earth termination.

(d) Services entering the structure above and below ground.(e) Radio and television receiving aerials.(f) Flag masts, roof level plant rooms, e.g. lift motor rooms, ventilating plant and boiler rooms, water tanks and

other salient features.(g) The construction of roofs to determine methods of fixing conductors with special regard to maintaining

weatherproofing of the structure.(h) Possible penetration of waterproofing membrane where earth terminations are to be sited beneath the

structure.

(i) The provision of holes through, or fixing to, reinforced concrete.(j) The provision of bonding connections to steel frame, reinforcement rods or internal metalwork, and for any

holes through the structure, parapets, cornices, and the like, to allow for the free passage of the lightningconductor.

(k) The choice of metal most suitable for the conductor, e.g. aluminium conductors for structures wherealuminium is employed externally.

(l) Accessibility of test joints; protection by non-metallic casing from mechanical damage or pilferage andhazard to persons; lowering of flagmasts or other removable objects; facilities for periodic inspection,especially on tall chimneys.

(m) The preparation of an outline drawing incorporating the foregoing details and showing the positions of themain components to form a basis for the record drawing recommended in Clause 8.4.

(n) Requirements for the coordination of the structures lightning protection earthing and the earthing of powerand communication services.

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FIGURE 4.3 TYPICAL LIGHTNING PROTECTION SYSTEM USING METAL IN OR ON A STRUCTURE

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FIGURE 4.4 TYPICAL SYSTEM EMPLOYING HORIZONTAL AND VERTICAL AIR TERMINATION NETWORK

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4.4.2.2 Route for conductors Conductors should be installed with a view to offering the least impedance to the passageof discharge current between the air terminals and ground. The most direct path is the best (see Clause 4.10.2). Theimpedance to earth is approximately inversely proportional to the number of widely separated paths, so that from eachair terminal there should be as many paths to earth as practicable. The number of paths is increased and the impedancedecreased by connecting the conductors to form a cage enclosing the building.4.4.2.3 Trouble-free installation Since a lightning conductor system, as a general rule, is expected to remain in workingcondition for long periods with little attention, the mechanical construction should be strong, and the materials usedshould offer resistance to corrosion.4.4.2.4 Economy of installation Economy of installation can be effected by keeping the variety of equipment to aminimum, avoiding the use of unusual air terminal ornaments and similar features, and taking advantage ofconstructional features of the building as far as practicable.

4.5 MATERIALS4.5.1 General Copper is recommended for its conductivity and durability; however, alternative materials may be usedif suitable for the environment in which they are installed and are otherwise satisfactory for the purpose (see Clause 4.6).Typical materials from which the component parts of lightning protection systems may be chosen are given in Table 4.2(see also Clause 4.5.2).Where insulating coatings are used, due regard should be given to their durability and non-flammability.For the protection of conductors at the tops of chimneys, see Clause 4.5.2.2(a).4.5.2 Corrosion4.5.2.1 Basic considerations The materials used in lightning protection systems should be resistant to corrosion resultingfrom the environment in which they are installed. This includes the effects of atmospheric, soil or water-borneelectrolytes or contaminants, and of contact with those metals or alloys which will lead to galvanic corrosion in thepresence of moisture.Corrosion resulting from contact of dissimilar metals can exist where a conductor is held by fixing devices on or againstexternal metal surfaces of a building or structure. Corrosion of this nature can also arise where water passes over arelatively cathodic metal such as copper carrying small amounts of copper corrosion product which is deposited as afine film of metallic copper on relatively anodic metals such as aluminium, zinc or steel. This causes destructive galvaniccorrosion of the latter metals which are commonly used in building cladding or roofing. The metallic components ofthe lightning protection system should therefore be compatible with the metals used externally on the structure overwhich these components pass or with which they may make contact.The components of lightning protection systems may be constructed from a variety of materials as described inClauses 4.5.2.2 and 4.5.2.3.4.5.2.2 Air terminations and downconductors Specific recommendations for air terminations and downconductors aregiven in Clauses 4.9 and 4.10 respectively. Account should be taken of the principles outlined in Clause 4.5.2.1 in theselection of materials for those components.Where there is a risk of metallic building elements being contaminated by corrosion products, e.g. from copperconductors, the use of insulated conductors should be considered. Such insulation may need protection against ultravioletradiation, e.g. by enclosure in conduit or by the application of appropriate paints or coatings.Where insulated cables are used as downconductors, bonding should be effected at the specified intervals and bondingconnections should be sealed against the ingress of moisture.Where structural steel or reinforcing bars form part of the downconductor system no further corrosion protection willnormally be required.With the common conductor materials, several specific precautions are necessary as follows:(a) Bare copper Copper should be of the grade ordinarily used for commercial electrical work.

NOTE: Where any part of a copper protective system is exposed to the direct action of chimney gases or other corrosive gases, it should beprotected by a continuous coating of tin, lead or other material suitable for the environment to which it is exposed. Such a coating should extendat least 500 mm below the top of the chimney. The coating should not be removed at joints.

(b) Bare alloys Alloys of metals should be substantially as resistant to corrosion as copper under similar conditions.Galvanized iron may be used as part or the whole of the downconductor system provided it has adequatecurrent-carrying capacity and is fastened with fittings having compatible corrosion characteristics. The galvanizediron may comprise the structural or decorative elements of the building subject to these requirements.

(c) Bare aluminium or aluminium alloys Care should be taken not to use aluminium in contact with concrete, mortar,the ground, or in other situations where moisture may be retained causing the aluminium to deteriorate. Precautionsshould be observed at connections with dissimilar metals.In aluminium lightning protection systems, copper, copper-covered and copper alloy fixtures and fittings shouldnot be used. Aluminium or aluminium alloy fixtures and fittings or non-metallic components of adequate strengthand durability are required. Special arrangements will be needed at the ground termination for this class of system.

Other materials may be used to the extent recommended elsewhere in this Standard.

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TABLE 4.2TYPICAL MATERIALS FOR CURRENT-CARRYING COMPONENTS

Material Standard Grade or typeCastings

Leaded gunmetalAluminium alloy

AS 1565AS 1874

C92410EA401 or AA607

Bars and rodsCopper, hard-drawn or annealedPhosphor-bronzeNaval brassAluminium bronzeAluminiumAluminium alloyGalvanized steelStainless steel

AS 1567AS 1567AS 1567AS 1567AS 1866AS 1866

AS 2837

1105184646271050

6063 or 6463A

TubesCopperGalvanized steel

AS 1432 or NZS 3501AS 1074

StripCopper, annealedAluminiumGalvanized steelStainless steel

AS 1566AS 1866

AS 1397 or NZS 3441AS 1449

1101200

Stranded conductorsCopper, hard-drawnAluminiumGalvanized steelStainless steel

AS 1746AS 1531.1AS 1222.1

Fixing bolts and screws for copperPhosphor-bronzeNaval brassAluminium bronzeCommon brassStainless steel

AS 1567AS 1567AS 1567

AS 2738.2AS 2837

518464627272303

Fixing bolts and screws for aluminiumand aluminium alloys

Aluminium alloyGalvanized iron or steel

BS 1473AS 1214

HB30

4.5.2.3 The earth electrode system The design of the earth electrode system should assume that the earth electrode willbe bonded, directly or fortuitously, to the following:(a) The multiple earthed neutral of the electricity supply (see AS 3000 or New Zealand Electrical Wiring Regulations).(b) The building structural steelwork or reinforcing material.(c) The communication service earth(s), if any.(d) The water supply pipes, if metallic.(e) Pipelines for gaseous or liquid fuels, if metallic.Some supply authorities attempt to isolate services (d) and (e) from (a), for galvanic corrosion control reasons, byinserting insulating spacers at the pipe entry. Consideration should be given to the fitting of surge arrestors across theinsulating spacers, in consultation with the supply authority, to prevent arc discharge without prejudicing the corrosioncontrol measures.The earth electrode system should be capable of satisfactory performance for the expected life of the lightning protectionsystem under the corrosion conditions existing at the site when bonded to(i) copper-based earthing systems (in most electrical installations);(ii) steel-based structural material;(iii) communication service earths which may be stainless steel, galvanized iron, copper or lead; and(iv) other metallic services, e.g. steel or copper pipes for water or gas.There are two hazards which arise from the bonding of other electrodes or service lines to the multiple earthed neutral(MEN) of the electrical supply. Firstly, if the earthing system of the electrical supply is copper-based (as is mostly thecase) it will cause progressive galvanic destruction of less cathodic metals, such as steel, to which it is connected.Secondly, the electricity supply has many loads connected to it that generate a direct current component; this directcurrent is an electrolytic hazard to other earthing systems to which the supply system earth is bonded. The amount ofdirect current which can be generated by each appliance is limited by AS 3100 and NZS 6200, but it is still sufficientto place at risk some types of electrodes. In particular, steel rods clad with copper or stainless steel suffer prematurefailure when this small amount of direct current perforates the cladding, initiating a process of self-destruction of therod core.

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It will be clear that the selection of any common metal or alloy for the earth electrode system places either itself or othersystems or services at some risk from galvanic corrosion.For lower-cost installations the use of one of the common metals or alloys may be satisfactory. A list of these, withcomments relating to their corrosion performance, is provided in Table 4.3.The extent to which the material combination can be damaging is related to soil moisture, the type and nature ofelectrolytes present, and area and resistance relationships. Inherently, if such materials are used, a maintenance checkingroutine is essential (see Paragraph B9, Appendix B).Where soil conditions are particularly aggressive from a corrosion viewpoint (soil resistivity typically below 30 .m,especially if combined with a pH value of less than 5.5), such as may exist in reclaimed marine areas, the use of an inertanode material (see AS 2832.1) may be necessary. Expert advice on the selection of an appropriate earth electrodesystem should normally be sought where such soil conditions exist.

TABLE 4.3CORROSION PERFORMANCE OF METALS AND ALLOYS USED

AS EARTHING ELECTRODES

Metal/alloyDeleterious effect of this metal/alloy

on other bonded undergroundferrous metals

Deleterious effect on thismetal/alloy from bonding to MEN

(copper-based) systemsGalvanized iron or steel Nil DamagingSolid copper Damaging NilCopper-clad steel Damaging Can be damaging

may be acceptableSolid stainless steel or nickel iron alloy Generally acceptable Can be damaging

may be acceptableStainless-steel-clad steel Generally acceptable Can be damagingBronze Generally damaging May be acceptableBrass Can be damaging May be acceptable

can be dezincifiedZinc Nil DamagingAluminium Nil Extremely damagingMagnesium Nil Extremely damaging

4.6 FORM AND SIZE OF CONDUCTORS4.6.1 Factors influencing selection The form and size of the conductors of the lightning protection system should beselected having regard to their(a) electrical and thermal characteristics (see Clause 4.6.2); and(b) mechanical strength, if required, and the likelihood of corrosion (see Clause 4.6.3).Typical dimensions of current-carrying components of lightning protection systems are given in Table 4.4.4.6.2 Electrical and thermal considerations Air terminations, downconductors and other conductors of the lightningprotection system which may carry the full lightning current should have a cross-sectional area and electricalconductivity such that they are able to carry the expected current without deterioration and without attaining temperatureswhich may give rise to risk of fire. Copper conductors having a cross-sectional area of at least 35 mm2 will normallybe necessary for this purpose. Conductors of other materials may be used provided they satisfy the above criteria forcurrent-carrying capacity and temperature rise.Conductors which, because of their arrangement in the lightning protection system, will carry only a proportion of thelightning current may have a cross-sectional area that is proportionately reduced but should be not less than one-fifthof the cross-sectional area needed to carry the full lightning current, or 6 mm2, whichever is the greater.Conductors of larger cross-sectional area than recommended above may be needed as indicated in Clause 4.6.3.4.6.3 Mechanical strength and corrosion considerations Conductors of larger cross-section than those recommendedin Clause 4.6.2 may be needed where(a) a significant reduction of cross-sectional area is likely to be experienced in service due to the effects of corrosion;

or

(b) an increase in cross-sectional area or section of different shape (e.g. tubular instead of solid) is required to provideadequate mechanical strength, e.g. for air terminations (see Clause 4.9.1).

Consideration should also be given to the use of a larger cross-sectional area than that recommended in Clause 4.6.2in situations where inspection or repair of the conductor is unusually difficult.

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4.7 JOINTS4.7.1 Effectiveness of joints The lightning protection system should have as few joints as possible. Joints and bondsshould be mechanically and electrically effective, e.g. clamped, screwed, bolted, crimped, riveted or welded. Whereoverlapping joints are used, the length of the overlap should be not less than 20 mm for all types of conductor. Contactsurfaces should first be cleaned then inhibited from oxidation with a suitable corrosion-inhibiting compound.4.7.2 Protective covering Joints and bonds may be protected with bitumen or embedded in a plastics compoundaccording to the local conditions. Particular attention should be given to joints of dissimilar metals.4.8 FASTENERS Conductors should be securely attached to the building or other object upon which they are placed.Fasteners should be substantial in construction and not subject to breakage, and should be, together with the nails,screws, or other means by which they are fixed, of the same material as the conductors, or of such nature that there willbe no serious tendency towards galvanic corrosion in the presence of moisture because of contact between the differentparts.Fasteners should be spaced so as to give adequate support to the conductor. Downconductors should be fastened atspacings not exceeding 1.0 m on horizontal runs and not exceeding 1.5 m on vertical runs.The method of fastening should not result in a reduction of the conductor cross-section below the minimumrecommended in Clause 4.6.

NOTE: Plastics materials may be used for the fixing of conductors provided such materials are suitable for long term exposure to the outdoorenvironment (e.g. stabilized against the harmful effects of ultraviolet radiation) and otherwise satisfy the recommendations of this Clause.

TABLE 4.4TYPICAL SECTION DIMENSIONS OF MAIN CURRENT-CARRYING COMPONENTS

Component Typical section dimensions (see Note)Air terminations

StripRodsStranded conductors

20 mm 3 mm10 mm dia.

35 mm2Downconductors

StripRodsStranded conductorsGalvanized materials

20 mm 3 mm10 mm dia.

35 mm235 mm2

Earthing electrodes and base conductorsHard-drawn copper rods for direct driving into soft groundHard-drawn or annealed copper rods for indirect driving or laying in groundGalvanized star stakesStainless steelGalvanized steel water pipeGalvanized steel or copper strip:

Base conductorsEarth electrodes

12 mm dia.10 mm dia.

25 mm 19 mm 19 mm10 mm dia.12 mm dia.

30 mm 5 mm30 mm 3 mm

Fixed connections (bonds)External:

StripRods

Internal:StripRods

20 mm 3 mm10 mm dia.

20 mm 1.5 mm6.5 mm dia.

Standard flexible connections (bonds)ExternalInternal

70 mm235 mm2

NOTE: Where stainless steel is used and is likely to carry the full lightning current, section dimensions larger than those indicated above may benecessary to avoid excessive temperature rise.

4.9 AIR TERMINATIONS

4.9.1 General requirements An air termination may consist of a vertical rod as for a spire, a single horizontalconductor as on the ridge of a small dwelling, or a system of horizontal conductors with vertical rods for the protectionof roofs of large horizontal dimensions (see Figure 4.4). Protection may also be provided with a horizontal overheadwire supported, if necessary, independently of the building to be protected or by a vertical air termination network (seeFigure 4.2). Salient points of the structure should be incorporated in the air termination network.

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The upper portions of the downconductors on tall buildings should be regarded as a continuation of the air terminationnetwork and should be positioned so as to intercept side strikes to the building. Preference should be given to placingdownconductors as near as possible to the exposed outer vertical corners of a building.

All metallic projections, on or above the main surface of the roof should be bonded to, and form part of, the airtermination network. In the case of aerials which have to be insulated from earth, a spark gap connection to earth orsurge arrestor should be provided.

If portions of a structure vary considerably in height, any necessary vertical air termination or air termination networkof the lower portions should, in addition to their own downconductors, be bonded to the downconductors of the tallerportions (see Figures 4.1 and 4.4).Air terminations may be of any form provided the section used and the means of attaching it to the building structurehave adequate mechanical strength to withstand the expected wind loading and natural harmonic resonances. Wherecopper rod is used as a vertical air termination, Table 4.5 gives guidance on the maximum height which should beadopted.

TABLE 4.5GUIDE TO MAXIMUM HEIGHT FOR VERTICAL AIR TERMINATIONS

COMPRISING COPPER RODSDiameter of rod

mmRecommended maximum height

m

10 < 16 16 < 21 21 < 26

1.01.52.0

26 < 31 2.5

4.9.2 Protection of roofs The parts of roofs most likely to be struck by lightning are parapets, the edges of flat roofs,chimneys, and the ridges and eaves of sloping roofs. Preference should be given to positioning the air terminations soas to protect these highly exposed parts.The height of a vertical air termination rod should be such that the tip will be not less than 0.5 m above the object tobe protected. On large flat and gently sloping roof areas a number of vertical rods of greater than 0.5 m in height maybe needed to establish a zone of protection over the whole roof area in accordance with Clause 4.2.Horizontal air termination conductors may be used to protect a planar roof surface. The roof will be deemed to beprotected by air termination conductors spaced no more than 6 m apart on the roof surface, provided that the highlyexposed edges or ridges forming the boundary of the surface are fully protected by the air termination network.Horizontal and vertical air termination conductors and interconnecting conductors of the air termination network shouldbe located so as to constitute, as nearly as local conditions permit, an enclosing network which joins each air terminationto each other and to all downconductors. Metal objects, such as gutters, should be bonded to the air termination network.In special circumstances, such as where it is desired to preserve the appearance of a historic building, the air terminationconductors may be installed immediately underneath the cladding (e.g. tiles) of a non-conductive roof. However, itshould be noted that, in the event of a lightning strike to the roof, the cladding will be punctured and may suffer somedamage.4.9.3 Protection of the sides of tall buildings4.9.3.1 Influence of forms of construction Many buildings will have perimeter columns in which the reinforcement (orstructural steel) is used as a part of the downconductor system. Where these columns on the external facade are nofurther than 10 m apart, no further protection will be required in respect of strikes to the side of the building.In the event of a strike to such a column or to isolated metal components such as small window frames, it is likely thata section of masonry cladding material may be dislodged.Where the risk of this is unacceptable, conductors should be installed on the external faces of the columns to receivethe strikes. These conductors will take the form of lightning air terminations/downconductors and should be bonded atthe bottom into the lightning protection system.Where suitable columns do not exist to receive strikes to the sides of buildings, vertical conductors should be installedfor this purpose. These conductors should be spaced around the perimeter of the building at a spacing not exceeding10 m or 30 m if the conditions of Clause 4.9.3.3 apply.4.9.3.2 Curtain wall construction It has become commonplace for tall buildings to have external glass curtain walls,with the curtain wall external to perimeter columns. The majority have major glass elements contained (and restrained)within a metallic framework. This framework is often inherently connected, electrically, to the metal in the buildingstructure via the standard connection details used to mechanically fix the curtain wall structure to the structural frameof the building itself.

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Where this inherent connection occurs and where the frame of the building is incorporated into the lightning protectionsystem, no further bonding of the curtain wall to the lightning protection system is necessary.Some curtain wall designs incorporate a metallic framework which is not exposed externally. This framework is thennot inherently available to receive direct strikes to the side of the building.For the curtain wall sections of this type above 45 m, where direct strikes to the side of the building are anticipated,special design and detailing modifications to the curtain wall should be made. The objective of these modificationsshould be to achieve a performance for receiving direct strikes equivalent to a curtain wall with an exposed framework.The modifications should provide exposed metalwork, suitable for receiving direct strikes, spaced at intervals of notmore than 10 m vertically and 10 m horizontally, or 30 m horizontally if the conditions of Clause 4.9.3.3 apply. Thisexposed metalwork should be located to occur particularly at corners of the curtain wall where the probability of directstrikes is the highest.The provision of exposed metalwork for this purpose at less than 45 m above ground is not necessary, however, thecurtain wall framework should be bonded to the lightning protection system at intervals not exceeding thoserecommended above.4.9.3.3 Dispensation for large flat surfaces For tall buildings, application of the rolling sphere in accordance withClause 4.2 will indicate that protection should be provided for the sides of the building above the height of the sphereradius (see Figure 4.1). However, large flat surfaces which are vertical or near vertical are less likely to form attachmentpoints for lightning discharges than are external corners or other projections which provide electric field enhancement.Consequently, notwithstanding the protection that may be inferred as necessary for such surfaces in accordance withClause 4.2, surfaces that are protected in accordance with the following recommendations will be deemed to be protectedfor the purposes of this Standard:(a) Downconductors should be provided on external corners and other external changes of direction where the plane

of the principal surfaces subtends an angle of greater than 20 (see sketch).

(b) Additional downconductors should be provided at the intervals necessary to comply with Clause 4.10.1.Surfaces that are inclined at 45 or more from the vertical should be treated as roofs and protected in accordance withClause 4.9.2.

4.10 DOWNCONDUCTORS4.10.1 Structures General The number of downconductors should be one for every 30 m of perimeter. The perimetershould be measured by the taut-string method (see Figure 4.4).A non-metallic structure exceeding 30 m in height should have at least two downconductors symmetrically spaced,bonded by a metal cap or by a conductor around the top.4.10.2 Route The route followed by downconductors should be in accordance with the following recommendations:(a) Downconductors should be distributed around the outside walls of the structure. It is undesirable to locate

downconductors in areas where persons are liable to congregate. The walls of light wells may be used for fixingdownconductors, but lift shafts should not be used for this purpose.

(b) Where the provision of suitable external routes for downconductors is impracticable or inadvisable, e.g. buildingsof cantilever construction from the first floor upwards, downconductors may be housed in an air space providedby a non-metallic, non-combustible internal duct. Any covered recess or any vertical service duct running the fullheight of the building may be used for this purpose, provided that it does not contain any unarmoured ornon-metal-sheathed service cable (see Clause 4.14.2.3).

(c) Any extended metal running vertically through the structure should be bonded to the lightning downconductor atthe top and bottom unless the clearances are in accordance with Clause 4.14.

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(d) A downconductor should follow the most direct path possible between the air termination and the earth termination.Right angle bends may be used when necessary but deep re-entrant loops should be avoided.

(e) A structure on bare rock, protected in accordance with Clause 4.12.3.1, should be provided with a